1. Field of the Invention
This invention relates to a fabricating method for a gate in a metal-oxide-semiconductor (MOS) transistor, and more particularly, to a fabricating method for a metal gate in a MOS transistor.
2. Description of Related Art
A MOS device includes stacked metal layers, oxide layers, and semiconductor layers wherein the metal layers mostly include aluminum, the oxide layers chiefly include silicon oxide, and the semiconductor layers mainly include silicon wafers. Because of the low melting point of aluminum, polysilicon, which is more similar to silicon, is used as a conducting material in a modern MOS device. As shown in FIG. 1, a polysilicon layer 102 working as a conducting layer has a contact with an oxide layer 104 over a silicon substrate 100, wherein the polysilicon layer 102 is further doped in order to increase its conductivity. A MOS device, as shown in FIG. 1, further includes source/drain regions 106 on either side of the polysilicon layer in the substrate 100.
As the design rule of a semiconductor device moves toward the sub-micron level, the thickness of the oxide layer under the gate of a MOS transistor is accordingly and necessarily reduced. In this case, the design of a conventional PMOS device no longer meets requirements, and is replaced by a PMOS design with a surface channel. The polysilicon gate in a MOS device having a PMOS with a surface channel is also changed from N-type used in a conventional design to a P-type, wherein the P-type polysilicon gate is normally formed by implanting boron ions into a polysilicon gate. Since the implanted boron ions tend to diffuse into the silicon substrate through the oxide layer located between the polysilicon gate and the substrate, and cause the instability of the threshold voltage of a MOS device, a metal gate is considered for use in a MOS device of a line width under 0.25 .mu.m.
Tungsten, which has a high melting point, is often used to form a metal gate. The gate can be formed by chemical vapor deposition (CVD) or physical vapor deposition (PVD), wherein the CVD-tungsten is a preferred material for a metal gate. Because fluorine ions within the WF.sub.6 vapor used in the process forming a tungsten layer by CVD cause impurities in the gate oxide, a titanium nitride layer has to be formed on the gate oxide before performing CVD.
As shown in FIG. 2, a titanium nitride layer 204 is formed on gate oxide 202, which is formed on a substrate 200, through CVD. Then, a CVD-tungsten layer 206 is formed on the titanium nitride layer 204, wherein the tungsten layer 206 and the titanium nitride layer 204 are later patterned to form a metal gate. Referring to FIG. 3, similarly, a titanium nitride layer 304 is formed on gate oxide 302, which is formed on a substrate 300, through PVD. Then, a CVD-tungsten layer 306 is formed on the titanium nitride layer 304, wherein the tungsten layer 306 and the titanium nitride layer 304 are later patterned to form a metal gate.
In the foregoing even though a titanium nitride layer 204 formed by CVD is capable of preventing the diffusion of the fluorine ions, the impurities within the titanium nitride layer 204 still cause the the gate oxide 202 to contain impurities in the follow-up processes. On the other hand the titanium nitride layer 304 formed by PVD, a columnar crystal structure, which is capable of making excellent contact with the gate oxide 302, is still unable to prevent the fluorine ions from diffusing into the gate oxide 302.